Nanoscale wires, also known as nanowires, have received considerable attention in recent years. One reason for such attention is the unique physical properties these one-dimensional structures can exhibit. For example, nanowires can exhibit quantum conductance and ballistic transport characteristics. In addition, one can use the size effect of nanowires as an effective means to tune the electronic and thermal properties of these materials.
The size effect includes two influences. The first influence is the confined dimension of a nanowire, which may modify the electronic band structure and phonon dispersion relationship, resulting in discrete electronic density of states and reduced phonon group velocity. This influence is defined as a confinement effect. The second influence is the high surface area present in nanowire structures, which introduces an increase in boundary scattering for both electrons and phonons. This influence can be defined as a surface effect.
The confinement and surface effects can be used to offer advantages over traditional bulk materials including: (1) enhancement of a parameter known as the Seebeck due to an increase of the density of states near the Fermi level; (2) increased carrier mobilities at a given carrier concentration due to quantum confinement and modulation doping; and (3) exploitation of the different length scales of phonon and electron scattering resulting in increased boundary scattering of phonons while effectively preserving electric carrier mobilities. These properties can have a beneficial effect on the development of high-efficiency thermoelectric materials.
Thermoelectric materials are important for power generation devices that convert waste heat into electrical energy. They can also be used in solid-state refrigeration devices. However, the widespread application of thermoelectric devices has been limited by their relatively low efficiency.
The efficiency performance of a thermoelectric device is defined by a dimensionless figure known as ZT, which is defined asZT=S2σT/Kt where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and Kt is the total thermal conductivity. ZT is roughly proportional to efficiency, wherein a ZT of 1 corresponds to an efficiency of approximately 10%.
Increasing ZT has proven difficult because the three parameters S, σ and Kt are all related to the free carrier concentration of electrons and thus are not independent. For example, doping of a material can increase a semiconductor's electrical conductivity, but also results in a decrease in the materials Seebeck coefficient and an increase in its thermal conductivity. Also, efforts to reduce the lattice thermal conductivity by alloying of the material also reduces the electrical conductivity by providing extra scattering mechanisms. Thus from the above equation, it can be seen that obtaining an enhancement in one quantity typically results in undesirable results in another quantity, and thus a failure to increase ZT overall.
In contrast to the difficulties in increasing ZT as stated above, the use of nanowires has provided a possible avenue for increasing ZT. However, methods developed thus far for the formation and growth of nanowires have used catalyst materials. The use of a catalyst in the growth of nanowires can result in contamination of the nanowire material and additional steps to remove the catalyst from the nanowires. Therefore, there is a need for a non-catalytic method for the formation and growth of nanowires.